We present the seasonal carbon (C) balance of the Mediterranean seagrass Posidonia
oceanica (L.) Delile calculated from seasonal rates of C gain (photosynthesis), C loss (respiration) and
growth. We compare our balance with the evolution of seasonal C reserves in order to determine the
parameters (shoot:root biomass, reserve allocation, photosynthetic parameters, etc.) that influence
the seasonal cycle of the plant. Additionally, we examine whether the annual C balance can be used
as a valid tool for testing the vulnerability of seagrasses to light reduction. The seasonal whole-plant
C balance showed alternate negative (from September to June) and positive (July and August) values.
This trend was the result of the interplay among several seasonal factors such as irradiance,
water turbidity, photosynthetic parameters, respiratory rates, shoot growth, within-shoot age distribution,
and principally, the low photosynthetic:non-photosynthetic biomass ratio. The lack of significant
correlation between seasonal growth and metabolic balance (C gain – C demand) did not permit
the prediction of plant growth. Conversely, the seasonal pattern of carbon storage was consistent
with the periods of positive and negative C balance. Consequently, reserve mobilization allows overwintering
and re-growth under conditions of negative C balance. Using different calculations the
annual C balance was found to be negative during 1993; this is in accordance with the carbohydrate
interannual depletion and the shoot density decline. Since Posidonia oceanica is regressing in the
Mediterranean, our carbon budget may notably contribute to future carbon models that can be essential
tools for defining the minimum light requirements for survival. More insight into the functioning
of some of the parameters that definitively influence this carbon budget (e.g.: the rhizome/root oxygen
consumption and the O2 to C conversion) is needed to fully understand the vulnerability of seagrasses
to light reduction.